Home, Chemistry, Physics

ISSR CLASSES
Checkpoint Science
iGCSE Chemistry
iGCSE Physics
iGCSE Coordinated Science
A-Level Chemistry

PRACTICALS
Practicals Home
Practicals A-Z
Study Plan for Practical Work

Thermal stability of group 2 nitrates

On heating sodium nitrate a thermal decomposition reaction took place in which sodium nitrite and oxygen gas were produced:

This is typical of the group 1 metal nitrates, which are relatively stable to heat. However, lithium nitrate decomposes more easily, producing brown nitrogen dioxide gas, oxygen and lithium oxide. We often find that the element at the top of the group behaves differently to the rest. This type of decomposition was also seen with copper(II) nitrate in an earlier experiment. It is also found on heating group 2 metal nitrates.

Observations for group 2 nitrates

Calcium nitrate is a white crystalline solid which quickly dissolves in its own water of crystallization to form a colourless solution. Droplets of colourless liquid are seen condensing at the mouth of the test tube. A white solid rapidly forms from the colourless solution. This solid then melts to give a colourless liquid, and a brown gas is seen shortly afterwards. A lot of brown gas is seen, and a glowing splint readily re-lights.

Strontium nitrateis a white solid which melts slowly to form a colourless liquid. You should have seen from the bottle that it is anhydrous (no water of crystallization) so there is no first dissolving stage here. Shortly after melting a brown gas is seen. Less brown gas is observed in the mouth of the test tube than before, and a glowing splint re-lights only with difficulty.

Barium nitrate is a white anhydrous solid which melts to give a dark green-brown liquid. After melting small amounts of brown gas are seen, and a glowing splint fails to re-light.

Theory

You should have seen from the video that the stability of the group 2 metal nitrates towards heat increases as we go down the group. This was the same trend that we observed with the stability of the group 2 metal carbonates in the last experiment. This is also a result of the increasing size of the cations down the group. The smaller cation causes a greater polarization in the nitrate ion and this reduces its stability:

The magnesium nitrate and calcium nitrate contained water of crystallization. You will see that the magnesium nitrate has no melting point given. This is because the compound decomposes before it melts. We can see in the video that it first dissolves in its own water of crystallization forming a colourless solution. Before all the water is driven off we can see brown gas produced as decomposition commences. The calcium nitrate can also be seen dissolving in its own water of crystallization, but this is first driven off to form the white anhydrous solid. This then melts and very shortly afterwards decomposition takes place. For the strontium nitrate and barium nitrate the anhydrous solids first melt, but there is an increasing delay between melting and reaching the decomposition temperature. Note also that it became increasingly difficult to generate enough oxygen gas to re-light the glowing splint. The equations are as follows:

The 1+ ions in group 1 are not sufficiently polarizing to cause the complete decomposition of the group 1 nitrates to form nitrogen dioxide gas (with the exception of the very small Li⁺ ion). This increased stability for group 1 and the bottom of the groups was the same pattern that we discovered for the group 1 & 2 carbonates.